"John Key has served New Zealand generously and with dedication. I wish him and his family the best for the future."

New Zealand First Leader Winston Peters said of the resignation: "The fact is that the economy is not in the healthy state that the Prime Minister has for so long claimed, and there are other issues which have caused this decision as well.

"The New Zealand public should have been informed of this a long time ago.

"Clearly the Prime Minister does not believe the superficial polls any longer.

"Contrary to certain perceptions the Prime Minister and his Finance Minister are unable to muddy the waters anymore."

Green Party co-leader Meteria Turei has said that whilst she fought against Key's politics, she "always supported his right to be a dad & a husband first".

I fought every day against John's politics but always supported his right to be a dad & a husband first. I wish him and his family well.

ACT Leader David Seymour also offered his support of the PM, congratulating him on his "noble" resignation.

"Under John's leadership, the Government has steadfastly maintained New Zealand's policy settings. As a result, we remain at the top of almost every international league table for good policy settings. In the long term, all Prime Ministers are judged for the policies they leave behind, and John will be judged well.

"It is a reality of MMP that ACT has played a vital role helping John to become and remain Prime Minister. He thanked me for that this morning. I'd like to thank him on behalf of ACT and its previous leaders for the constructive way we've worked together over the past eight years.

"We also extend our warmest regards to Bronagh as the Keys get their lives back after a decade of service to the country."

Kiwis have taken to social media to express their shock at the news of Key's resignation.

Others are calling his resignation "fishy" and are seeking answers.

Councilor for Auckland's North Shore Richard Hills responded to the news on Facebook saying this year is all about "shock politics".

On Twitter, people compared the National party leader's resignation to the announcement, made within hours of Italian Prime Minister Matteo Renzi's similarly unexpected resignation.